The present invention relates to a composite electroacoustic transducer for reproducing medium and high frequencies.
In the field of sound systems for high fidelity, the importance of the distance between the emission centers of the transducers used to reproduce the various frequencies is known. This distance can in fact cause significant alterations in the frequency response of the entire sound system. This is substantially due to the fact that in the so-called crossover regions, i.e. where the frequency ranges intersect one another, the transducers assigned to reproducing adjacent frequency ranges, for example medium and high ones (otherwise known as midranges and tweeters) simultaneously emit the same frequency with the same intensity.
For the sake of greater clarity, FIG. 2 illustrates the case in which the listening point Po is in any point of the plane M which is median with respect to the segment which joins the two emission centers P1 and P2. In this case, the distances of the listening point Po from said emission centers P1 and P2 are identical, so that the sound waves produced by the transducers arrive at said point Po at the same time and mutually in phase. This produces a 6 dB increase in sound pressure level with respect to the level produced by a single transducer.
If vice versa, as shown in FIG. 3, the listening point Po is on a plane N which is different from said median plane M, the respective distances from the emission centers P1 and P2 are different, so that the sound waves produced by the transducers arrive at said point Po at different times and with different phases. The maximum phase opposition (180.degree.) occurs for all positions of the listening point Po in which the difference between said distances is equal to half the wavelength of the emitted frequency. In this case, the two emissions cancel each other out, producing a deep attenuation in frequency response.
Finally, in this case, if the frequency of the applied signal is changed in a continuous manner, one observes a first significant attenuation in frequency response at the frequency whose half-wavelength is equal to the difference between the distances of the emission centers from the point Po, followed by a series of successive attenuations which repeat at odd multiples of the emitted frequency, as shown in the frequency response chart shown in FIG. 4.
Ultimately, the combination of the effects produced by the distance of the listening point from the emission centers of the transducers and by the range of frequencies reproduced by both of said transducers causes a change in the frequency response of the sound system as a whole and a difficult reconstruction of the sound image to obtain a correct stereophonic effect.
In order to reduce this problem, the emission centers of the midrange and of the tweeter are usually placed as close as possible to each other. However, this solution has a physical limit constituted by the bulk of the magnetic assemblies of the electroacoustic transducers used.
Therefore, conventional transducers of the above described type often have a non-optimum sound quality and have relatively large dimensions and a proportionately high weight. This constitutes an evident limitation, especially in the fields of application in which reduced bulk and weight are required, for example for high-fidelity systems to be installed in motor vehicles and the like.